1. Field of the Invention
The present invention relates to piezoelectric fibers, articles comprising piezoelectric fibers, methods for making piezoelectric fibers and electronic devices using piezoelectric fibers.
2. Background
Piezoelectricity refers to a phenomenon observed in some materials in which imposition of a stress will establish an electric field whose intensity is proportional to the stress level. This phenomenon is credited to Jacques and Pierre Curie who discovered piezoelectricity in quartz in 1880 (Curie, P. J. and J. Curie, Crystal Physics-Development by Pressure of Polar Electricity in Hemihedral Crystals with Inclined Faces. Acad. Sci. (Paris) C.R. Hebd Seances, 1880. 91: p. 294), but the materials most often in use today as piezoelectrics are barium titanate (BaTiO3) and lead zirconate titanate (PZT). Both are ceramic materials which require high temperature processing in the presence of a high electric field in order to render them piezoelectric. They tend to be expensive and brittle materials. (See Rosen, C. Z., B. V. Hiremath, and R. E. Newnham, eds. Piezoelectricity. 1992, American Institute of Physics: New York for a review of piezoelectricity.)
Piezoelectric materials exhibit a linear coupling between a stress field and an electric field. Equations show that piezoelectricity works either with a mechanical field inducing an electrical one, or vice versa. Generally transduction from a mechanical signal to an electrical signal is referred to as sensing, while transduction using electrical input to produce a mechanical output is referred to as actuation. In order to fully investigate piezoelectric materials, it is necessary to consider both sensing and actuation because some measurements are easier to make on sensors and others on actuators.
BaTiO3 and PZT, were first discovered in the late 1940s and early 1950s (Jaffe, B. 1955: U.S. Pat. No. 2,708,244, issued May 10, 1955; Gray, R. B. 1949: U.S. Pat. No. 2,486,560, issued Nov. 1, 1949). Efforts since then to find new piezoelectric materials generally have met with disappointment. The most promising development was the discovery of piezoelectricity in PVDF, but this polymer loses its piezoelectricity at a relatively low temperature (70° C.) and requires uniaxial or biaxial stretching in order to introduce piezoelectricity (Kawai, H., The Piezoelectricity of Poly(vinylidene Fluoride). Jpn. J. Appl. Phys., 1969. 8: p. 975). Mechanical fatigue is also a problem with PVDF. Few commercial products using piezoelectric PVDF have been marketed although the military has employed thick PVDF hydrophones.
Recently, there has been work on piezoelectricity in polypropylene foam, often written as LDPP for low density polypropylene. (See Gerhard-Multhaupt, R. Voided polymer electrets-New materials, new challenges, new chances. in 11th International Symposium on Electrets. 2002 for a review.) LDPP is produced in a blow-extrusion process that results in polypropylene with closed cell spherical voids. The material is then biaxially stretched to produce disk-shaped voids. It is exposed to corona charging at levels of about 20 kV that cleaves the molecular bonds of the gas trapped in the voids yielding a d33 of up to 300 pC/N. LDPP has a couple inherent problems that will likely limit its ultimate application in transducers. First, it loses its piezoelectric function starting at about 50° C. This means that the material is inappropriate for any use that will cause significant warming (potentially any operation in air, for instance). Second, at high pressures, it is likely the relatively low stiffness of the air voids compared to the polymer will result in collapse of the voids, possibly with discharging. Thus it is not appropriate for high pressure use.
Composites formed by placing a piezoelectric material in a polymer matrix have also been pursued successfully for many years. The bulk of the work has been on 1-3 composites, in which rods of piezoelectric materials (PZT or BaTiO3) are embedded in a polymer matrix. Applications of piezoelectric 1-3 composites have focused on sonar although there has been increasing interest in their use as well for nondestructive evaluation of structures and acoustic monitoring of faults in the nuclear industry (Fleury, G. and C. Gondard, Improvements of Ultrasonic Inspections through the Use of Piezo-Composite Transducers. Transducer Workshop, 1996). Compared to the standard piezoelectric materials, 1-3 composites are lower mass and more rugged. Volume fractions of the ceramic component vary from 0-50% with thicknesses ranging from fractions of a millimeter to 25 millimeters (Benjamin, K., Recent Advances in 1-3 Piezoelectric Polymer Composite Transducer Technology for AUV/UUV Acoustic Imaging Applications. J. Electroceramics, 2002. 8: p. 145). The material typically is produced using an injection molding process to produce ceramic rods in a pattern with a plate structure at one end to keep the rod spacing and alignment fixed. A polymer then fills the regions between the rods, and the plate end is sliced off.
Piezoelectric materials are the key components of electromechanical transducers (sensors and actuators) for automatic control systems, and measurement and monitoring systems. Electromechanical transducers have become ubiquitous in our world, being found in everything from hearing aids to automobiles, from clothing dryers to perimeter sensors, and from elevators to computers. The history of transduction reads like a time line for materials invention with each new coupling mechanism discovery leading to new devices (Busch-Vishniac, I. J., Electromechanical Sensors and Actuators. 1999, New York: Springer). The common characteristic of electromechanical sensors and actuators is that they are electrical at one end and mechanical at the other. The linking of these two energy domains occurs typically through exploitation of electromechanical coupling phenomena in materials.
The most common acoustics transducers are microphones and loudspeakers. They are found in every telephone, in tape and digital audio recorders, and increasingly in automobiles, where they are being used for hands-free communication and in monitoring engine performance. Today, most common microphones are electret microphones. Electret materials are those which exhibit a permanent polarization or space charge. First reported in 1962 (Sessler, G. M. and J. E. West, Self-Based Conderser Microphone with High Capacitance. J. Acoust. Soc. Am., 1962. 34: p. 1787), electret microphones use a membrane suspended under tension above a rigid backplate, a perforated backplate and back cavity to reduce stiffness, and a small hole through the structure for dc pressure equalization.
By contrast, a piezoelectric microphone can be much simpler in structure. The piezoelectric material serves as the dielectric element, with a metal surface on top and bottom. It is unnecessary to supply any tension, to vent the device, or to provide a back cavity and perforated backplate. The result is a very simple microphone in which the material is contained either in a ring allowing sound access from both sides (a gradient microphone) or in a cylinder closed at one end (conventional pressure microphone). While it is possible to make piezoelectric microphones from BaTiO3 and PZT, they are generally less sensitive and more expensive than electret microphones.